1. Field of the Invention
The present invention relates to a device and method for controlling the changing operation of an on-load tap changer that changes the tap of a transformer.
2. Description of the Related Art
It has been well known that the on-load tap changer changes the tap of a transformer in a load operation state to change and adjust the turn ratio of secondary winding/primary winding of the transformer, in other words, changes and adjusts the transformation ratio.
The on-load tap changer is driven by a motor operation mechanism to operate. The mechanism responds to a remote actuation command from an automatic voltage regulating relay (normally provided in the main control room) that continuously monitors the voltage state of the load of the transformer or from an operator who monitors the operation state in the main control room. The mechanism then operates the on-load tap changer to regulate voltage at the secondary side of the transformer for a dropped amount in the secondary voltage if for example the voltage of the load connected to the secondary side of the transformer drops.
The operation of the on-load tap changer necessitates bridging between taps during the changing operation, and a current limiting resistor is employed in a circuit to limit the bridging current between the taps during the bridging operation. This type of device is called “resistor type on-load tap changer.”
According to conventional techniques, the current interruption with the resistor type on-load tap changer is normally successful during one-tap changing operation if the following conditions are satisfied. These ideas have been adopted as a Japanese standard for on-load tap changers (on-load tap changer JEC2220) and an international standard (IEC-214), and performance testing methods, assessments and the like have been established.
1) Interrupting current for use is at a commercial frequency of 50 Hz or 60 Hz.
2) Current is interrupted at a current zero point.
If tap changing operation is carried out in the above described conditions, the on-load tap changer can normally interrupt current and successfully make a connection to a target tap.
A conventional on-load tap changer therefore regulates voltage to a prescribed level at the load side of a transformer provided that the current interruption conditions in the above 1) and 2) are satisfied. Therefore, the changing operation is carried out in response to an actuation command from the automatic voltage regulating relay or the operator as described above without monitoring for the presence/absence of a current zero point in the waveform of current actually passing through the on-load tap changer or measuring the cycle of the current zero point.
However, if the load connected to the transformer is special, for example if power is supplied to a load such as an AC electric railcar or a flicker, or if a plurality of on-load tap changing transformers are operated in parallel, the following disadvantage is encountered.
For a special load, for example, the supplied current may become current with a distorted waveform including a harmonic component and may not pass a current zero point at predetermined intervals unlike a normal commercial frequency.
If the on-load tap changer is operated by current with such a distorted waveform because of the special load, the interrupting current does not pass a current zero point in the cycle of a commercial frequency. If the cycle is prolonged in particular, the duration of arc discharge from the opening to the extinction of the arc is prolonged, so that the arc discharge could continue for not less than 10 ms that is the period tolerated for arc generation that can normally be interrupted, in other words, the arc cannot be extinguished within a normal changing period. At worst, if the arc cannot be extinguished before the main contact is closed on the next tap side, the arc current is passed, which could give rise to a serious accident such as short-circuiting between the taps.
When a plurality of on-load tap changing transformers are provided and operated in parallel with a normal load (operating at a commercial frequency) and power is supplied to the load, even slight time difference in changing operation between the on-load tap changers can generate one tap differential voltage at the parallel arrangement of transformers, and circulating current superposed with DC current is passed between the transformers arranged in parallel because of the tap differential voltage.
The on-load tap changers are adjusted by driving shafts that couple the motor operation mechanisms that drive the on-load tap changers to the on-load tap changers so that the on-load tap chargers operate in timing as similar as possible in order to reduce the time to pass of the circulating current as much as possible and the temperature rise of the transformers caused by the circulating current as much as possible.
However, it would be difficult to adjust the plurality of tap changers to be in the same timing for structural reasons, and very small time difference for changing operation is tolerated.
Therefore, the circulating current caused by the slight operation timing difference is superposed to diverted load current to each of the transformers and the resulting current is passed to each of the on-load tap changers. The superposed current includes a DC component as disclosed by JP-A-2005-12954 and therefore is transient current that does not pass a current zero point. If one preceding on-load tap changer operates and then a succeeding on-load tap changer operates to interrupt current with slight time difference from the operation of the preceding on-load tap changer during the period before the DC component is attenuated to the level of normal current that passes a current zero point, the current interruption may depart from the current interruption conditions, and the current may not be interrupted normally by the main contact on the interrupting side, which could cause current to be passed with continuous arc discharge for a significant time period. At worst, the current could continue to be passed before the main contact is closed on the next tap side, which could result in a serious accident such as short-circuiting between taps.
According to conventional techniques, however, no specific countermeasure has been suggested to prevent an on-load tap changer from interrupting current with a distorted waveform generated in an environment for a special consumer load (that generates the current waveform of a harmonic component) or current that does not pass a current zero point and is in a cycle other than that of a commercial frequency and no specific countermeasure has been suggested to prevent each on-load tap changer from interrupting transient current superposed with DC current caused by the operation timing difference between the on-load tap changers included in the transformers when the transformers are operated in parallel.